personally, I don't think Dumbledore does anything without a purpose. He knew what he was doing when he awarded the three all those points, he was setting them up as heroes, and encouraging them to keep on risking their lives in order to prepare to fight Voldemort and company.  He also didn't care if he insulted the Slytherins or not, since he tended to favor his own House.  Look at how he used Severus and disregarded completely the fact that Sirius and James nearly killed him at the Shrieking Shack, no wonder why Snape was so bitter. 

That was why I decided to even the score and give the Slytherins their due for once.

I had to have Harry approach Dumbledore and ask him for the extra points for Slytherin, he's the only one Dumbles would have listened to and it corrected a major injustice.  Dumbledore always claimed to never favor his own house, but he lied, since he allowed the Marauders to get away with everything and never punished them, and he put Harry and his friends in danger more than once by sitting back and watching.  He also deliberately encouraged the rivalry between Gryffindor & Slytherin by never rewarding or praising the Slytherins for anything.